FlightSafetys are all different, and of the ones I've been to some were better than Simuflite, some were worse. In the end, it all comes down to the quality of the instructors, and both FSI and Simuflite have good and bad instructors. Hopefully you will get a good one!
I have to say that I think that Simuflite's books and especially checklists are far, far, superior to FlightSafety's.
One difference I have noted, across a couple of programs, is that Simuflite takes the oral portion of the type rating exam (or recurrent, for that matter) much more seriously than FSI. Expect to be grilled, and to be able to recite the memory items exactly as written, etc.